Performances at the iconic venue in Greenwich, south-east London, had previously been classed as ‘low risk’ but security chiefs have since classified all events as ‘medium risk’.

Tightened entry controls and extra security marshals have been hired across the venue as a result.

Security briefings for staff before all major events have also been increased from just 10 minutes to an hour and a half to include strict instructions on what to do in the event of a major incident.

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Notices on official invitations for the Brit Awards state: “There will be increased security at the O2 this year, so please allow sufficient time as all vehicles and guests will be subjected to longer waiting times getting into the show.”

An insider said: “Every precaution necessary is now taken with security and the Brits will have its biggest every security detail enforced.

“Since the attacks in Paris every aspect of security was looked at and nothing will be left to chance.”

The same stringent security measures have been employed at arenas in Liverpool, Manchester, Newcastle and Leeds.

Hosts: Ant and Dec will do the honours (Image: JM Enternational)

The Mirror can also reveal how other sporting and entertainment venues across Britain have drafted in ex-special forces personnel to run security operations.

A ring of steel has been placed around stadiums and arenas as security services and the police warn of a major assault by Islamic State militants.

Premier League and Championship football grounds have also bolstered security including ground sweeps and extra bomb searches.

Within hours of the atrocities on November 13 in the French capital, security companies across the country were inundated with requests to supply specialist staff capable of orchestrating emergency plans in the event of a mass casualty attack.

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Hundreds of bar and nightclub staff around the UK have received ‘specialist counter-terror training’ in the event of a ‘co-ordinated gun and bomb attack’, just like the assault on the French capital.

A leaked document from the National Counter Terrorism Security Office warns of the likelihood of a ‘no warning, multiple and coordinated terror attack’ at entertainment venues in Britain.

Marcus Gentles, security expert and trainer, said: “We are certainly seeing an increase in the amount of clients who want their staff to be trained in matters of counter terrorism.

“We train staff to be on the look out for all manner of explosive devices including suicide vests, chemical bombs, car bombs and even IEDs (improvised explosive devices), which would normally be found on the battleground.

“But every situation must be prepared for in today’s climate.

On alert: The O2 Arena (Image: Getty)

“Any of the big arenas will take on these methods with some employing another structure in their security run by Close Protection Operatives.

“These people could be ex-military or ex-special forces and will have a heightened sense of what to look out for when dealing with counter-terrorism issues or indeed how to co-ordinate an emergency in the event of a terrorist attack.”

A spokesman for the O2 in London, said: “The O2 operates an extremely thorough security regime which is risk based.

“We have a regular dialogue with the UK security services and, after the events in Paris at the end of 2015, we reviewed our security operations across the venue.

“We will continue to enforce the enhanced security put in place, as well as increase our general venue security operations at this time.

“Venue security is monitored at all times to ensure the safety of all our visitors and we ask that they allow extra time for security checks when entering the venue.”